Datasheet

LTC3553
20
3553fb
OPERATION
BUCK REGULATOR
Introduction
The LTC3553 includes a constant-frequency current-
mode 200mA buck regulator. At light loads, the regulator
automatically enters Burst Mode operation to maintain
high effi ciency.
Applications with a near-zero-current sleep or memory
keep-alive mode can command the LTC3553 buck regula-
tor into a standby mode that maintains output regulation
while drawing only 1.5µA quiescent current. Load capability
drops to 10mA in this mode.
The buck regulator is enabled, disabled and sequenced
through the pushbutton interface (see the Pushbutton
Interface section for more information). It is recommended
that the buck regulator input supply (BVIN) be connected
to the system supply pin (V
OUT
). This is recommended
because the undervoltage lockout circuit on the V
OUT
pin
(V
OUT
UVLO) disables the buck regulator when the V
OUT
voltage drops below the V
OUT
UVLO threshold. If driv-
ing the buck regulator input supply from a voltage other
than V
OUT
, the regulator should not be operated outside
its specifi ed operating voltage range as operation is not
guaranteed beyond this range.
Output Voltage Programming
Figure 3 shows the buck regulator application circuit. The
output voltage for the buck regulator is programmed using
a resistor divider from the buck regulator output connected
to the feedback pin (BUCK_FB) such that:
V
BUCK
= 0.8V•
R1
R2
+1
⎛
⎝
⎜
⎞
⎠
⎟
Typical values for R1 can be as high as 2.2M.
(R1 + R2) can be as high as 3M. The capacitor C
FB
cancels the pole created by feedback resistors and the
input capacitance of the BUCK_FB pin and also helps
to improve transient response for output voltages much
greater than 0.8V. A variety of capacitor sizes can be used
for C
FB
but a value of 10pF is recommended for most
applications. Experimentation with capacitor sizes between
2pF and 22pF may yield improved transient response.
Normal Buck Operating Mode (STBY Pin Low)
In normal mode (STBY pin low), the buck regulator per-
forms as a traditional constant-frequency current mode
switching regulator. Switching frequency is determined
by an internal oscillator which operates at 1.125MHz. An
internal latch is set at the start of every oscillator cycle,
turning on the main P-channel MOSFET switch. During
each cycle, a current comparator compares the inductor
current to the output of an error amplifi er. The output of
the current comparator resets the internal latch, which
causes the main P-channel MOSFET switch to turn off and
the N-channel MOSFET synchronous rectifi er to turn on.
The N-channel MOSFET synchronous rectifi er turns off
at the end of the clock cycle, or when the current through
the N-channel MOSFET synchronous rectifi er drops to
zero, whichever happens fi rst. Via this mechanism, the
error amplifi er adjusts the peak inductor current to deliver
the required output power. All necessary compensation
is internal to the buck regulator requiring only a single
ceramic output capacitor for stability.
At light load and no-load conditions, the buck automatically
switches to a power-saving hysteretic control algorithm that
operates the switches intermittently to minimize switching
losses. Known as Burst Mode operation, the buck cycles
Figure 3. Buck Regulator Application Circuit
3553 F03
0.8V
V
IN
C
FB
C
OUT
V
BUCK
MP
MN
GND
BUCK_FB
R1
SW
L
PWM
CONTROL
EN
STBY
R2